Conventionally, as a lid for an underground structure, there is known a type of a flat receiving structure which is configured such that an outer diameter of a lid body is slightly smaller than an inner diameter of a receiving frame; an outer peripheral portion of the lid body and an inner peripheral portion of the receiving frame are substantially vertical planes; and a bottom surface of the lid body is supported by a shelf portion projecting from an inner periphery of the receiving frame. There is also known a type of a gradient receiving structure which is configured such that the outer diameter of the lid body is equal to the inner diameter of the receiving frame, and the outer peripheral portion of the lid body and the inner peripheral portion of the receiving frame are formed to have a steep gradient by reducing a diameter toward a downward direction such that the lid body bites into the receiving frame.
However, in a lid for an underground structure having a flat receiving structure, the configuration that the outer diameter of the lid body is slightly smaller than the inner diameter of the receiving frame results in a gap between the outer peripheral portion of the lid body and the inner peripheral portion of the receiving frame. For this reason, as a result of vehicles repeatedly travelling on the lid for an underground structure, there may be a case that the bottom surface of the lid body and the upper surface of the shelf portion are worn away, thereby causing rattling, vibration, and noise of the lid body.
In contrast to this, as a lid for an underground structure of a gradient receiving structure, Patent Document 1 describes a configuration in which inclination angles with respect to the vertical line of a gradient surface of the outer peripheral portion of a lid body and a gradient surface of the inner peripheral portion of a receiving frame are limited to 5° to 10°, and the height of the gradient surface of the outer peripheral portion of the lid body is smaller than the height of the gradient surface of the inner peripheral portion of the receiving frame. According to the configuration described in Patent 1, by limiting the angles of gradient surfaces as described above, it is possible to ensure a sufficient biting force of the lid body against the receiving frame, thereby significantly suppressing rattling, vibration, and noise of the lid body, and is also possible to achieve an effect that sliding up of the lid body seldom occurs.